Unlock Characters
Press [Y] at the custom character screen, then enter the following codes:

phayder - Unlock Alanna (Necris)
jihan - Unlock Ariel (Iron Clad)

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Achievements
Complete each achievement to get the allotted gamerscore:

Ace (10) - Get the "Top Gun" award in 20 matches.
Armadillo (10) - Get the "Roadkill" award in 10 matches.
Around the World (20) - Win a multiplayer match on every map.
Bag of bolts (30) - Complete Chapter 3 in Insane.
Being a Hero (10) - Return 100 Orbs in Warfare matches.
Flag waver (20) - Completed 100 CTF matches played to at least 3 captures.
Get a life... (50) - Get 200 kills in multiplayer on 50 different days.
Big Game Hunter (10) - Get the "Big Game Hunter" award in 10 matches.
Brain Surgeon (10) - Get the "Head Hunter" award in 10 matches.
Connect the dots (20) - Complete 100 Warfare matches played to at least 3 points.
Deathwish (10) - Get "Bullseye" (self-destruct to destroy an enemy) award in 20 matches.
Delivering the Hurt (10) - Have over 20 minutes of "UDamage Time".
Does not compute (30) - Complete Chapter 3.
Don't taze me bro! (10) - Get the "Combo King" award in 10 matches.
Equal opportunity destroyer (20) - Win a Ranked match in every game type.
Fear the Reaper (60) - Defeat Akasha and complete the campaign in Insane.
I need some backup (10) - Complete 1 mission in co-op.
I see how it is (10) - Complete an Instant Action match in every game mode.
Goo God (10) - Get the "Biohazard" award in 10 matches.
Got your back (20) - Complete 10 missions in co-op.
Hammerhead (10) - Get the "JackHammer" award in 10 matches.
Hat trick (20) - Achieve a Hat Trick in 10 CTF or VCTF matches.
Soldier's blood (30) - Complete Chapter 2.
Spice of life (20) - Play a match with every mutator, using only 1 per match.
Strongest Link (10) - Get the "Shaft Master" award in 10 matches.
Survival of the fittest (10) - Have over 10 minutes of "Invulnerable Time".
Like the back of my hand (40) - Collect every power up on every map.
Lock and Load (10) - Complete Chapter 1.
Have a nice day! (20) - Get the "Rocket Scientist" award in 10 matches and "Flak Master" award in 10 matches.
I'm not on a holy war (60) - Defeat Akasha and complete the campaign.
Off to a good start (10) - Get the "First Blood" award in 40 matches.
Open war (30) - Complete Chapter 4 in Insane.
Jack of all trades (10) - Kill an enemy with every vehicle (except hoverboard).
Just business (30) - Complete Chapter 2 in Insane.
Just warming up (10) - Finish playing a "Versus" Ranked match.
Killjoy (10) - End sprees in 20 matches.
Lets get it on (20) - Complete a multiplayer match in every game type.
Never Saw it Coming (10) - Have over 20 minutes of "Invisibility Time".
Shard-o-matic (10) - Get the "Blue Streak" award in 10 matches.
Sir Slays a Lot (20) - Get the "Monster Kill" award in 20 matches.
Not in Kansas any more (30) - Complete Chapter 4.
Paint the town red (20) - Complete 100 Deathmatch or Duel matches played to at least 20 kills.
Pistolero (10) - Get the "Gun Slinger" award in 10 matches.
Ranked Champion (50) - Win 500 Ranked matches.
Seeing Red (10) - Have over 20 minutes of "Berserk Time".
Serial killer (20) - Get the "Killing Spree" award in 20 matches.
Thanks to all the little people (40) - Complete a campaign in co-op.
Thirty minutes or less (20) - Complete 100 VCTF matches played to at least 3 captures.
Untouchable (20) - Reach 20 kills against a god-like bot in instant action without dying.
Winning is the only thing (10) - Finish with the highest points in a Ranked match.

Akasha - Kill Akasha in the final mission.
Damian - Unlocked in Chapter 4 of Campagin mode.
Kragoth - Unlocked in Chapter 4 of Campagin mode.
Lauren - Always select the second choice for all missions. Kill Lauren in Deathmatch in Chapter 2 of Campaign mode. Lauren will eventually become unlocked.
Loque - Unlocked after a Capture The Flag match in Chapter 5.
Malakai - Unlocked in Chapter 4 of Campagin mode.
Matrix - Always select the second choice for all missions. Kill Matrix in Deathmatch in Chapter 3 of Campaign mode. Matrix will become unlocked.
Play as Alanna (Necris) - At the custom character screen, press Y, then enter "phayder" as a code to unlock Alanna (Necris).
Play as Ariel (Iron Clad) - At the custom character screen, press Y, then enter "jihan" as a code to unlock Ariel (Iron Clad).
Scythe - Always select the second choice for all missions. Kill Scythe in Deathmatch in Chapter 4 of Campaign mode. Scythe will eventually become unlocked.

Biohazard: Berserk Room
On Biohazard, there is a Berserk power-up past a Bio Rifle and some stairs Just before the stairs is a green button. If you shoot the green button, it will turn red. Anyone inside the Berserk room will be locked inside, and there will be green toxic waste that can kill you.

Spinning Hoverboard
Anytime you are using the hoverboard, hold down the [Right Analog-stick] and press the [Left Analog-stick Left] or [Right] to spin in that direction.